Mirror lock-up
Most dSLRs have a feature known as "Mirror Lock-Up" (MLU). Normally, the mirror inside the SLR body flips up immediately before the shutter opens. This movement causes "mirror slap", vibration caused by the mirror reaching the end of its travel and hitting the underside of the viewfinder. This vibration can cause blurring of the image as the camera body shakes from the impact.
Mirror lock-up is a mode of operation where the mirror inside the camera body flips up and is held there for a period of time (to allow the vibration to stop) before the shutter is opened.
Mirror slap blurring is most usually observed when long lenses are in use. It does however also affect relatively short focal lengths if the exposure time is long. Attached are two (100%) crops of the same scene, taken one after the other. Both are tripod mounted, using a carbon-fibre tripod which supposedly has better vibration damping than aluminium. Both are the same exposure/shutter speed/ISO settings, both are taken at the same focal length of 55mm. One image is taken just with a gentle finger on the button, the other is taken using the self timer and mirror lock-up. I was surprised by how much shake there was given the short focal length used.
Lesson learned:
For still-life, use MLU and self timer or MLU and a remote release, even at shorter lens lengths.
Duncan.
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